Auto-start X application from read-only file system

Question

In order to protect the SD card in an application which may suffer a sudden loss of power I am attempting to mount the root in read-only mode.

In the working (read-write) system our full-screen application gets launched by the automatic X log-in executing an LXDE auto-start script.

I've got the system running, mostly, by the previously suggested /etc/fstab table:

proc            /proc            proc    defaults                   0       0
/dev/mmcblk0p1  /boot            vfat    defaults                   0       2
/dev/mmcblk0p2  /                ext4    defaults,noatime           0       1
tmpfs           /tmp             tmpfs   defaults,noatime,mode=1777 0       0
tmpfs           /var/log         tmpfs   defaults,noatime,mode=0755 0       0
tmpfs           /var/lock        tmpfs   defaults,noatime,mode=0755 0       0

My problem lies in getting the X application up and running. LXDE complains about not being able to create an .xauthority file. So I added the following kludge (note the access mode!):

tmpfs           /var/lib/lightdm tmpfs   defaults,noatime,mode=1777 0       0

Unfortunately the auto-start still does not work, leaving me staring at the lightdm-greeter screen. Attempting to actually log in swiftly returns me to the same prompt, and curiously the logs seem to be silent about what the failure might be.

My next attempt was to forgo the window manager entirely through a custom .xsession or .xinitrc. While this works when manually executing startx it does not log on automatically for the pi user. I fear my attempts to fiddle with the global /etc/X11/xinit/xinitrc also met with little success.

The next idea was to replace the default lightdm-greeter script by linking to the auto-start script in /usr/share/xgreeters and editing /ect/ligthdm/ligthdm.conf with alternate greeter-user and greeter-session settings. This fails with a "Greeter closed communication channel" message. Presumably there is a protocol of some sort which a proper greeter is expected to implement.

Any suggestions? I suspecft this is child's play to anyone with actual experience of Unix administration but it's got me bashing my head against a wall.

Answer 1

I did several things. It autostarts with the account kiosk

cat /home/kiosk/kioskstartup
#!/bin/sh
xset -dpms
xset s off
unclutter &
matchbox-window-manager -use_cursor no -use_titlebar no  &
midori -e Fullscreen -a http://127.0.0.1:4079/index.html

cat /home/kiosk/.profile
# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.

# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022

# if running bash
if [ -n "$BASH_VERSION" ]; then
    # include .bashrc if it exists
    if [ -f "$HOME/.bashrc" ]; then
    . "$HOME/.bashrc"
    fi
fi

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi
xinit ./kioskstartup

cat /usr/share/xsessions/matchbox.desktop
[Desktop Entry]
Encoding=UTF-8
Name=Matchbox
Comment=This session logs you into Matchbox
Exec=matchbox-session
Terminal=False
Icon=
Type=Application

In addition to that, I modified the affected startup scripts so they weren't writing to /var, and instead created /tmp directories and put data there...

    chmod 777 /tmp/run
fi

if [ ! -d /tmp/cache ]
then
    mkdir /tmp/cache
    chmod 777 /tmp/cache
fi

if [ ! -d /tmp/cache/lighttpd ]
then
    mkdir /tmp/cache/lighttpd
    chmod 777 /tmp/cache/lighttpd
fi
if [ ! -d /tmp/log ]
then
    mkdir /tmp/log
    chmod 777 /tmp/log
fi
if [ ! -d /tmp/log/lighttpd ]
then
    mkdir /tmp/log/lighttpd
    chmod 777 /tmp/log/lighttpd
fi

PATH=/sbin:/bin:/usr/sbin:/usr/bin
DAEMON=/usr/sbin/lighttpd
NAME=lighttpd
DESC="web server"
PIDFILE=/tmp/run/$NAME.pid
SCRIPTNAME=/etc/init.d/$NAME

DAEMON_OPTS="-f /etc/lighttpd/lighttpd.conf"

test -x $DAEMON || exit 0

set -e

check_syntax()
{
    $DAEMON -t $DAEMON_OPTS > /dev/null || exit $?
}

if [ "$1" != status ]; then
    # be sure there is a /tmp/run/lighttpd, even with tmpfs
    # The directory is defined as volatile and may thus be non-existing
    # after a boot (DPM §9.3.2)
    if ! dpkg-statoverride --list /tmp/run/lighttpd >/dev/null 2>&1; then
        install -d -o www-data -g www-data -m 0750 "/tmp/run/lighttpd"
    fi
fi

. /lib/lsb/init-functions

case "$1" in
    start)
    check_syntax
        log_daemon_msg "Starting $DESC" $NAME
        if ! start-stop-daemon --start --oknodo --quiet \
            --pidfile $PIDFILE --exec $DAEMON -- $DAEMON_OPTS
        then
            log_end_msg 1
        else
            log_end_msg 0
        fi
        ;;
    stop)
        log_daemon_msg "Stopping $DESC" $NAME
        if start-stop-daemon --stop --retry 30 --oknodo --quiet \
            --pidfile $PIDFILE --exec $DAEMON
        then
            rm -f $PIDFILE
            log_end_msg 0
        else
            log_end_msg 1
        fi
        ;;
    reload|force-reload)
    check_syntax
        log_daemon_msg "Reloading $DESC configuration" $NAME
        if start-stop-daemon --stop --signal INT --quiet \
            --pidfile $PIDFILE --exec $DAEMON
        then
            rm $PIDFILE
            if start-stop-daemon --start --quiet  \
                --pidfile $PIDFILE --exec $DAEMON -- $DAEMON_OPTS ; then
                log_end_msg 0
            else
                log_end_msg 1
            fi
        else
            log_end_msg 1
        fi
        ;;
    reopen-logs)
        log_daemon_msg "Reopening $DESC logs" $NAME
        if start-stop-daemon --stop --signal HUP --oknodo --quiet \
            --pidfile $PIDFILE --exec $DAEMON
        then
            log_end_msg 0
        else
            log_end_msg 1
        fi
        ;;
    restart)
    check_syntax
        $0 stop
        $0 start
        ;;
    status)
        status_of_proc -p "$PIDFILE" "$DAEMON" lighttpd && exit 0 || exit $?
        ;;
    *)
        echo "Usage: $SCRIPTNAME {start|stop|restart|reload|force-reload|status}" >&2
        exit 1
        ;;
esac

exit 0

Last thing, I totally got rid of LXDE and went to the window manger above. I used lighttpd instead of monkey as web server because it supports aliases properly. My browser is midori.

This config needs a little bit of work, but it does work!

Answer 2

Try to sudo chmod 777 /tmp and also /var too if something else fails.

In my case it was the only extra thing needed to do.

Answer 3

One thing which is important to understand is that the filesystem doesn't get corrupt from being rw at the time of a crash. It gets corrupt because of an unfinished write operation at the time of a crash. If no write operation is in progress, your rw filesystem is as safe as an ro one.

So I suggest you leave your filesystem rw, enable journalling and stop worrying about it. If the only write operation that still remains is creating the .xauthority file at boot time, chances that a crash will happen exactly at that moment are already pretty slim, and even in that case journalling will most probably handle the issue without you even noticing it.

Personal anecdote:

I have an SBC running headless (not exactly your case, but still) which is used as a wireless router. My mom which is obsessive about electricity pulls the plug every evening and plugs it back in the morning. The root filesystem is rw, yet the thing has been running for months now without any issues.